It is an object of the present invention to provide decholesterolized, defatted egg powder having a lower cholesterol content and fewer calories, and yet having good flavor and physical properties comparable to those of raw whole egg, and which is safe for use as a food because of the absence of any remaining solvent.
Eggs, particularly hens' eggs, are eaten in the form of fried eggs, boiled eggs or scrambled eggs, and they are used frequently as materials for processing various cakes or foods such as raw cream, mayonnaise, cake raw materials, etc. It is well known that they are high calorie protein-aceous food having such a mix of amino acids beneficial to man that they are one of the basic protein foods.
It is also well known that hens' egg yolks are high in cholesterol (1600 mg cholesterol/100 g yolk). Cholesterol is a compound having an aromatic polycyclic structure and a hydroxyl group. It is biosynthesized in bodies of animals and also taken into the bodies from foods. Many versatile and important functions of cholesterol have been recognized, such as digestion of lipids, involvement in in vivo transport or metabolism of lipids in general, and a wetting effect on the skin in cooperation with phospholipids.
As the result of recent improvements as well as westernization of the diet, egg consumption in Japan is rapidly growing. In western countries which are large egg consumers, an increase of cholesterol concentration in human blood due to a large cholesterol intake in food has already caused problems.
It is now generally accepted in the field of medicine, notwithstanding its important functions, that when the concentration of cholesterol in the blood increases, it deposits gradually on the inner walls of blood vessels or encroaches into the walls of blood vessels in a form combined with lipoproteins in the blood and becomes a major cause of arteriosclerosis, a disease of the aged, and as a consequence leads to hypertension and cardiac diseases.
Under such circumstances it is common in the United States for individuals to limit their consumption to five eggs a week.
In response thereto several kinds of low fat decholesterolized, defatted egg products have appeared in the market. They are classified into two major groups. One of them is a pseudo-defatted egg in which the yolk has been removed from the beginning. As the name indicates they are egg imitations in which egg white or the like is used as a major protein material, and various ingredients are admixed in order to make their composition and flavor close to those of real eggs. Although these egg imitations actually do not contain cholesterol, and they are indeed low calorie foods, their flavor, their nutritive values and their physical properties are of quality considerably different from raw whole eggs because yolks are not used, and hence they have not been accepted by consumers nutriologically and psychologically.
The other group uses whole egg as a raw material and is produced by a solvent extraction method to remove cholesterol and total lipids.
These products have been subjected to defatting and decholesterolization with a hydrophilic solvent for extracting oil and fat, such as chloroform-methanol or ethanolacetone-ethyl ether. Thus there is obtained defatted whole egg having an almost white color and containing hardly any cholesterol, oils, or fats.
Although these ought to have produced defatted whole eggs having high nutritive value because their raw material is raw whole eggs, such products are still imperfect and have not yet enjoyed popularity because their proteins have been denatured and there is considerable loss in flavor, taste and performance when used as eggs, as compared with raw whole eggs.
The following conclusions have been reached as to the causes therefor:
First of all, what is considered to be the cause is the use of a non-polar-polar solvent mixture for extraction of cholesterol and oils and fats. Although a non-polar solvent is indispensable for extraction of oils and fats, it was believed that thorough extraction of oils and fats from eggs or the like containing a large amount of water could not be achieved with a non-polar solvent alone because of its hydrophobic nature. Accordingly, an alcoholic type hydrophilic polar solvent is usually admixed to break, for example, hydrophobic bonds and electrostatic bonds between protein and phospholipids, such as those which are connected to membranes of living bodies and are relatively difficult to extract. By such solvents, cholesterol and total fats are completely removed and thus the crude fat content is reduced to about 0.1 percent, and phospholipids are almost totally absent from the residue of extraction.
That is to say, because of complete defatting, proteins in the defatted whole egg obtained by this method are denatured to a considerable extent. Furthermore, phospholipds themselves are absent. Hence, marked degradations are noticeable even after defatted proteins are thoroughly rehydrated, not only in the reduction in solubility, but also in various properties such as foaming and oil-retention as well as emulsifying.
The second cause is a problem of treatment after extraction. Heat treatments, such as that involved in removing the extraction solvent, result in denaturing of the protein which promotes degradation and loss of protein.
Solvents usually used for defatting, such as hexane or the like, must be completely removed from proteins after extraction. Thus, a process is carried out in which residual proteins are contacted with steam to drive off the remaining hexane, but heating carried out at a temperature of 100.degree. C. or higher for a long period of time, however, utterly changes the macro-structure of proteins to such an extent that the change is clearly observable even with the naked eye, making rehydration of the defatted proteins very difficult.
In order to overcome the above-mentioned two drawbacks, a new solvent extraction method has recently been developed. This improved method is directed only to yolk. Namely it comprises conducting cholesterol removal and partial defatting of yolk solid matter only with a non-polar solvent such as hexane, heptane, cyclohexane and the like. Since denaturing of egg proteins was so extensive in prior methods, attention was paid to the removal of moisture in the raw material, and the whole process was carried out at a relatively low temperature to control denaturing of proteins.
Although this method succeeded in maintaining egg properties to a certain extent, the solvent was not thoroughly removed because of the lower temperatures employed in the solvent removal step to prevent protein denaturation. Thus there are available only defatted whole eggs, from which beneficial physical properties and functions possessed by raw whole eggs have been lost because of too severe thermal denaturation, or excessively defatted whole eggs which can hardly be used as food because of the harmful residual solvent.
On the above mentioned background, the inventors of the present invention conducted various studies about methods for treating raw whole eggs from which all neutral fats and cholesterol can be extracted without incurring heat denaturation, i.e. without heating raw whole eggs, but while retaining a part of phospholipids in order that protein denaturation does not occur, and solvent does not remain. As the result of these studies the instant process has been invented.